Tubing and duct systems for conveying fluids are in widespread use in many industries. In the aerospace industry, for example, welded ducts are used in environmental control systems and in wing de-icing systems for conveying heated air from the engine to leading edges and nacelle inlet nose to prevent ice from forming on those surfaces in icing conditions in flight. Tubing, of course, may be utilized in many other industries as well including the oil industry or other industries in which transport elements are required for use in extreme environments at times.
Tubing materials may be manufactured using superplastic forming (SPF) and diffusion bonding (DB) techniques. For many years it has been known that certain metals, such as titanium and many of its alloys, exhibit superplasticity. Superplasticity is the capability of a material to develop unusually high tensile elongations with reduced tendency toward necking. This capability is exhibited by a few metals and alloys and within a limited temperature and strain rate range. Titanium and titanium alloys have been observed to exhibit superplastic characteristics equal to or greater than those of any other metals. With suitable titanium alloys, overall increase in surface areas up to 300% are possible, for example. Advantages of superplastic forming are numerous including abilities to create complex shapes and deep drawn parts, and low deformation stresses are required to form the metal at the superplastic temperature range thereby permitting forming of parts under low pressures that minimize tool deformation and wear.
Diffusion bonding (DB) refers to metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a time duration so as to cause co-mingling of atoms at the joint interface. Diffusion bonding can be accomplished entirely in the solid-state at or above one-half the base metal melting point (absolute). Actual times, temperatures, and pressures will vary from metal to metal. The joining surfaces are brought within atomic distances by application of pressure. Adequate pressure is provided to cause some plastic flow to fill normal void areas. If pressures are too low, small voids may remain at the joint inter face and the joint strength will be less than the maximum obtainable. The application of pressure also breaks up the surface oxides and surface asperities so as to present clean surfaces for bonding. Elevated temperatures used for diffusion bonding serve the functions of accelerating diffusion of atoms at the joint interfaces and providing a metal softening which aids in surface deformation so as to allow more intimate contact for atom bonding and movement across the joint interface. Elevated temperature and application of pressure also results in diffusion of the surface contaminants into the base metal during bonding which allows metal atom-to-atom bonding and thereby strengthens the bond. Sufficient time is allowed to ensure the strengthening of the bond by diffusion of atoms across the joint interface.
According to existing processes, tubing sheets are manufactured in a flat form, in which one or more superplastically formable metal sheets are placed in a die cavity defined between cooperable dies, the sheets are heated to an elevated temperature at which the sheets exhibit superplasticity, and then a gas is used to apply differential pressures to the opposite sides of the sheets in order to form the sheets. The pressure is selected to strain the material at a strain rate that is within its superplasticity range at the elevated temperature, stretch the sheets, and cause the sheet to assume the shape of the die surface. In this way, the sheets can be formed to a complex shape defined by the dies.
In other existing processes, SPF and DB can be performed in a combined forming/bonding operation. For instance, in one example combined SPF/DB process, three metal sheets are stacked in a flat form to form a pack. A stop-off material is selectively provided between the sheets to prevent portions of the adjacent surfaces of the sheets from being bonded. The pack is heated and compressed in a die cavity with sufficient gas pressure so that the adjacent portions of the sheets that are not treated with the stop-off material are joined by diffusion bonding. Thereafter, a pressurized gas is injected between the sheets to inflate the pack, and thereby superplastically form the pack to a configuration defined by the surface of the die cavity. Such a combined SPF/DB process can be used, e.g., to produce complex honeycomb sandwich structures that are formed and diffusion bonded to define hollow internal cells. Generally, the simplicity of the superplastic forming and/or diffusion bonding processes can result in lighter and less expensive structures with fewer fasteners and higher potential geometric complexity.
However, using existing superplastic forming and diffusion bonding processes results in flat sheets that still require further manufacturing processes to form the sheets into a tubing system, which may alter the bonds created. Using examples described herein, a titanium (or other alloy) double-walled tube can be manufactured using SPF and DB that is corrosion resistant, heat resistant, and structurally reinforced with increased cross section for greater strength than seen in flat sheets to provide a fail safety for a titanium tube.